Photoreflectance (PR) and piezoreflectance (PzR) of substrates such as silicon-germanium (SiGe), gallium arsenide (GaAs), indium gallium sulfate (InGaS) exhibit a typical band gap of approximately 1.5 eV or 3.5 eV.
Rapid PR (RPR) spectroscopy requires the simultaneous detection of a small modulated reflectance (ΔR) signal and a large time-invariant, unmodulated reflectance signal R, and the relation of the small modulated reflectance and unmodulated reflectance (ΔR/R). Even at parts of the spectrum where it is significant, ΔR is relatively small compared to the unmodulated reflectance R, of the order of several to hundreds parts per million. Various sources of optical and electrical noises will be present and these dominate the ΔR signal. However, the ΔR signal is always present at the known modulation frequency and so methods of frequency-discriminating signal recovery are typically employed to detect a signal at this known frequency. Therefore, phase sensitive lock-in amplification was required for the measurement of the ΔR signal in PR spectroscopy and the measurement was to be made at different times for each wavelength, so that the ΔR/R spectrum was therefore generally recorded in a serial spectral mode. This limits the practical speed of the PR measurement and precludes its widespread industrial application in high-speed production line inspection of semiconductor wafers.
Strained silicon (sSi) refers to silicon in which strain is engineered locally in a device structure or globally across a wafer by a local or global stress to accelerate electrons, which allows manufacture of faster devices. Faster sSi transistors due to increased electron mobility and velocity have already been proven. As a result, the technology of strain engineering is being widely used to speed carrier mobility in transistor channels in order to increase the drive currents.
Globally sSi on a wafer comprises a very thin layer of single-crystal silicon strained by pseudomorphic growth up to a critical thickness on a relaxed SixGe1-x stressing layer of wider lattice constant dependent on the Ge mole fraction x.
In all global sSi technologies with one exception, the strain in the silicon is described as biaxial, the result of two effects, namely, the expansion of the silicon lattice due to the wider lattice constant of the relaxed Si1-xGex layer, which is tensile stressing it, and the contraction of the silicon lattice in the vertical direction because of its behavior as a near-perfect Poisson solid. These two strain effects, hydrostatic tension, and uniaxial compression, will be presently shown to be mirrored by two competing effects in the electronic band structure in the vicinity of the E1 critical point of silicon, whose combined effect is measurable by PR spectroscopy. Such a sSi layer can be transferred onto an SiO2 buried oxide layer while retaining the strain to form a strained silicon-on-insulator (SSOI) wafer.
X-ray diffractometry (XRD) and Raman spectroscopy (RS) have been used for strain measurement in silicon. XRD suffers from excessive measurement time, and RS has poor resolution. Other optical inspection techniques look for slip-lines on the surface or cracks in the edge of silicon wafers. Optical inspection methods detect defects rather than measuring strain and are therefore unsuitable for identifying problems before defects occur, controlling strain sources, relating strain defects to specific process parameters, or managing strain levels in wafers.
Consequently, it would be advantageous if an apparatus existed that is suitable for directly measuring strain in a wafer production process in time to apply the measurements to prevent defects.